1. Field of the Invention
The present invention pertains to an apparatus adapted to be lowered into a wellbore for selectively generating multiple controlled explosions to provide seismic signals suitable for exploration for mineral values.
2. Background
In exploration for subterranean mineral values, one technique which has proven successful is known generally as vertical seismic profiling. This exploration process involves placing a geophone sonde in a borehole and generating seismic signals on the surface or, in the case of an inverted vertical seismic profile, the seismic signal is generated in the borehole and an array of geophones is placed on the earth's surface. The latter technique has been successfully used to collect seismic data as discussed in a paper entitled, "An InSitu Analysis of 3-D Seismic Lateral Resolution" presented at the 55th Annual Society of Exploration Geophysicists Meeting in 1985, by the authors, Charles C. Mosher and Ian A. Mason.
One disadvantage with conventional inverted vertical seismic profiles is the time required to set up and fire a seismic signal generator. Conventional borehole seismic signal sources may utilize an explosive source such as a length of Primacord explosive. Although the Primacord type explosive can be easily tailored to minimize or prevent damage to the wellbore and its casing, the time required for winching the support mechanism for the explosive to the surface, reloading the explosive and lowering the explosive to the next point of signal generation is disadvantageous to the above-mentioned type of exploration process. Moreover, not only is it desirable to avoid the time consuming process of retrieving and reloading the support for the seismic signal source, but it is also desirable to move the signal source within a borehole to obtain data from various depths or locations with respect to the signal-receiving devices. It is toward the objectives of overcoming the disadvantages of prior art inverted seismic profiling processes and toward enjoying the advantages of having a multiple shot, downhole seismic signal source that the present invention has been directed as will be appreciated by those skilled in the art.